1. Field of the Invention
The present invention relates to control of combustion in an internal-combustion engine. The present invention notably relates to a method for detecting an abnormal combustion, of pre-ignition type at low speed and high load, in a combustion chamber of such an engine, and particularly relates, but not exclusively, to such a method applied to a downsized spark-ignition engine running at very high loads.
2. Description of the Prior Art
Spark-ignition engines have the advantage of limiting local emissions (HC, CO and NOx) thanks to the excellent match between the operating mode (at fuel/air ratio 1) and their simple and low-cost post-treatment system. Despite this essential advantage, these engines are badly positioned in terms of greenhouse gas emissions because Diesel engines with which they compete with, can reach 20% less CO2 emissions on average.
The combination of downsizing and supercharging is one of the solutions that have become increasingly widespread for lowering the consumption of spark-ignition engines. Unfortunately, the conventional combustion mechanism in these engines can be disturbed by abnormal combustions. This type of engine includes at least one cylinder comprising a combustion chamber defined by the inner lateral wall of the cylinder, by the top of the piston that slides in the cylinder and by the cylinder head. Generally, a fuel mixture is contained in this combustion chamber and it undergoes compression, then combustion under the effect of a spark ignition, by a spark plug. These stages are grouped together under the term “combustion stage” in the rest of the description.
It has been observed that this fuel mixture can undergo various combustion types and that these combustion types are the source of different pressure levels, and of mechanical and/or thermal stresses some of which can seriously damage the engine.
The first combustion, referred to as conventional combustion or normal combustion, is the result of the propagation of the combustion of a fuel mixture compressed during a prior engine compression stage. This combustion normally propagates in a flame front from the spark generated at the plug and there is no risk it may damage the engine.
Another combustion type is a knocking combustion resulting from an unwanted self-ignition in the combustion chamber. Thus, after the fuel mixture compression stage, the plug is actuated so as to allow ignition of this fuel mixture. Under the effect of the pressure generated by the piston and of the heat released by the fuel mixture combustion start, a sudden and localized self-ignition of part of the compressed fuel mixture occurs before the flame front resulting from the ignition of the fuel mixture by the spark plug comes near. This mechanism, referred to as engine knock, leads to a local pressure and temperature increase and it can generate, in case it occurs repeatedly, destructive effects on the engine and mainly on the piston.
Finally, another combustion type is an abnormal combustion due to a pre-ignition of the fuel mixture before the spark plug initiates ignition of the fuel mixture present in the combustion chamber.
This abnormal combustion affects in particular engines that have undergone downsizing. This operation is intended to reduce the size and/or the capacity of the engine while keeping the same power and/or the same torque as conventional engines. Generally, this type of the engine is essentially of gasoline type and it is highly supercharged.
It has been observed that this abnormal combustion occurs at high loads and generally at low engine speeds, when timing of the fuel mixture combustion cannot be optimum because of engine knock. Considering the high pressures and the high temperatures reached in the combustion chamber as a result of supercharging, an abnormal combustion start can occur, sporadically or continuously, well before ignition of the fuel mixture by the spark plug. This combustion is characterized by a first flame propagation phase that occurs too soon in relation to that of a conventional combustion. This propagation phase can be interrupted by a self-ignition involving a large part of the fuel mixture present in the combustion chamber which is much larger than in the case of engine knock.
In cases where this abnormal combustion takes place repeatedly, from engine cycle to engine cycle, and starts from a hot spot of the cylinder for example, it is referred to as “pre-ignition”. If this combustion occurs suddenly, in a random and sporadic way, it is referred to as “rumble”.
The latter abnormal combustion leads to very high pressure levels (120 to 250 bars) and to a thermal transfer increase that may cause partial or total destruction of the moving elements of the engine, such as the piston or the piston rod. This pre-ignition type is currently a real limit to spark-ignition engine downsizing. It is a very complex phenomenon that can have many origins. Several hypotheses have been mentioned in the literature to explain its appearance, but so far none has been clearly validated. It even rather appears that several of these potential causes occur simultaneously and interact with one another. This interaction, the violence of the phenomenon and its stochastic character make its analysis extremely complicated. Furthermore, all the various studies on the subject come up against the problem of proper identification of these abnormal combustions. It is in fact difficult to say if an engine is more sensitive than another to pre-ignition as long as one cannot reach a decision on the nature of each of the combustions within a given sample.
A method allowing detection and to quantify these abnormal combustions is therefore absolutely essential because it precisely allows establishing this hierarchy and to identify approaches that will enable to improve the design and the adjustments of the engines. This operation is particularly interesting during test bench engine developments.
The general methodology for dealing with these abnormal combustions is diagrammatically shown in FIG. 1, with first a prevention phase (PP) for limiting to the maximum phenomenon appearance risks, then a detection phase (PD) when prevention is not sufficient to avoid the phenomenon, to determine whether it is pertinent to intervene in the very cycle where pre-ignition was detected by means of a corrective phase (PC).
The detection phase comprises a signal acquisition stage, then a signal processing stage allowing detection of the appearance of pre-ignition at high load, to characterize and to quantify it.
EP Patent Application 1,828,737 describes a method for detecting the appearance of pre-ignition at high load, of a rumble type. This method is based on the measurement of a signal relative to the progress of the combustion and a comparison with a threshold signal. The presence of an abnormal combustion of the rumble type in the combustion chamber is detected when the amplitude of the signal significantly exceeds that of the threshold signal. According to this method, the threshold signal corresponds to the amplitude of the signal produced upon knocking combustion or normal (conventional) combustion.
However, according to this method, the achieved detection does not allow action during the detection cycle per se. The corrective actions on this type of pre-ignition can only be carried out after such a phenomenon has occurred, which may seriously harm the engine integrity.
Another method is also described in French Patent 2,897,900. According to this method, action can be taken more rapidly after pre-ignition detection. One can act during the same cycle as the phenomenon detection cycle. The threshold signal is therefore first calculated with a computer, before engine operation and then stored in data charts of the computer referred to as maps.
However, the use of engine maps does not allow detection any time, that is in real time at the start of such a phenomenon. Detection may therefore occur too late. Furthermore, no quantification of the evolution of the phenomenon can be carried out. Thus, the necessity or not of applying a corrective phase is based only on the comparison of two amplitudes at a given time. Now, such a phenomenon may also start, then stop without causing any damage to the engine, and therefore require no corrective phase.